In radio frequency (RF) applications, a conventional Doherty amplification circuit typically includes a main RF amplifier coupled in parallel with a peaking RF amplifier. At low power levels, the main RF amplifier in the conventional Doherty amplification circuit is activated and biased for linear operation, while the peaking RF amplifier is deactivated. The peaking RF amplifier is activated once an RF signal reaches a particular signal level, which is generally at or near a compression point of the main RF amplifier. To increase power efficiency, quarter wave transmission line transformers or quarter wave transmission line inverters are often employed in conventional Doherty amplification circuits in order to provide the appropriate impedance transformations while the peaking RF amplifier is activated and deactivated. Unfortunately, quarter wave transmission line transformers/inverters have narrowband characteristics and thus do not allow for broadband operation. Furthermore, at the higher frequencies, the quarter wave transmission line transformers/inverters in these conventional Doherty amplification circuits degrade the power efficiency of the Doherty amplification circuit at backed-off power levels. Generally, this is due to the narrowband characteristics of the quarter wave transmission line transformers/inverters and, in addition, to the inability of the quarter wave transmission line transformers/inverters to correct for parasitic effects in the peaking RF amplifier at higher frequencies.
Accordingly, RF circuit designs that improve bandwidth performance and/or the power efficiency of the Doherty amplification circuit are needed.